In the case of synthesizing various chemical products industrially in chemical plants, it has been known that by-products other than the aimed products are formed or unreacted products are remained. For example, in a polymerizing reaction plant, an apparatus for separating a resultant polymer and an unreacted monomer in a reaction vessel is essentially provided and the recovered unreacted monomer is usually utilized again as a raw material for the polymerizing reaction. In the polymerizing reaction, while it has also been known that multimers such as oligomers are by-produced, in the case where the multimer remains in the polymer, since this results in a drawback that chemical products having aimed properties cannot be obtained, or the properties of the chemical products are degraded with time, a separating and removing step for the multimer is often provided. While the unreacted monomer can be made re-utilizable by merely incorporating it into a raw material supply line, the multimer cannot be handled in the same manner as the monomer, and therefore, it is subjected exclusively to incinerating treatment or discarding treatment.
Furthermore, not only in the polymerizing reaction plants but also in chemical plants for synthesizing low molecular compounds, by-products or multimers such as dimers or trimers of the aimed compounds are formed and it is thus necessary to separate by-products and multimers from the aimed compounds, for example, by a distillation. In the case of the low molecular compounds, although separation of by-products is relatively easy, since it is often difficult to separate the multimer and the aimed compound, the multimer and the aimed product are often contained in the wastes such as distillation residues. There are scarce methods of effectively utilizing these wastes and, since only the incineration or discarding treatment is applied, this has resulted a problem in view of the resource saving.
On the other hand, it has been attempted in recent years to detoxify wastes or obtain effectively utilizable products by using hydrolysis or oxidizing reaction in water at high pressure and high temperature. For example, it has been proposed a method of detoxifying liquid wastes by utilizing an oxidizing reaction in a supercritical or subcritical state (for example, refer to Patent Document 1), a method of hydrolyzing various high molecular compounds by using water in a supercritical or subcritical state (for example, refer to Patent Document 2) and, further, a method of obtaining pure terephthalic acid and glycol from polyethylene terephthalate wastes (for example, refer to Patent Documents 3 or 4).
However, while the method proposed in Patent Document 1 is important as the detoxifying method, since it accompanies the oxidizing reaction, this results in a problem for the usefulness of the obtained materials. Further, there is no description in any other patent documents about a method of efficiently decomposing and recovering compounds containing oligomers or dimers or higher multimers, or a decomposing and recovering apparatus.
On the other hand, while methods of decomposing isocyanate compounds and recovering them as amine compounds (for example, refer to Patent Documents 5, 6, 7 or 8) have been disclosed, since each of them is a batchwise method, enormous energy for cooling or heating as well as depressurization or pressurization upon replacement of batch is required and the processing amount is also limited in view of the scale of the apparatus, and therefore, it cannot be said that they are industrially suitable.
Then, it has been proposed a method for continuously decomposing and recovering an isocyanate compound and an apparatus for decomposing and recovering them, in which multimers or other by-products of isocyanate compounds by-produced in production lines of isocyanate compounds as target compounds to be hydrolyzed is selected among the wastes in the chemical plants for which no other treating methods than incineration or discarding are present, and the isocyanate compounds are decomposed and recovered as a raw material or a derivative thereof as intermediate material thereby enabling the effective re-utilization thereof (hereinafter referred to also as a conventional example).
In the followings, the conventional example is described with reference to the appended drawings. FIG. 4 is a schematic explanatory view showing an example of an apparatus for decomposing and recovering an isocyanate compound, which is for practicing a method for decomposing and recovering the same according to the conventional example. The apparatus for decomposing and recovering the isocyanate compound includes a longitudinal cylindrical reactor 51 that brings water at high pressure and high temperature into contact with an isocyanate compound thereby causing decomposition reaction. A water supply line 52 intervened with a supply pump 52a for pressurizing water and a heater 52b for heating the water at high pressure pressurized by the supply pump 52a is in communication to the bottom of the reactor 51. Furthermore, a compound supply line 53 intervened with a supply pump 53a for pressurizing the isocyanate compound in a molten state or liquid state, and a heater 53b for heating the isocyanate compound in a molten state or liquid state pressurized by the supply pump 53a is in communication with lateral side near the lower portion of the reactor 51.
Moreover, a decomposition reaction product discharge line 54 for supplying a decomposition reaction product formed by decomposition at a temperature of the reactor 51 of from 190 to 300° C. is in communication with a dehydrating column 55 as a dehydrating device from the decomposition reaction product discharge port at the top of the reactor 51. Further, a decomposition reaction product supply line 56 for supplying a decomposition reaction product which is dehydrated as well as removed with CO2 in the dehydrating column 55 is in communication with a depressurized distillation column 57 as a purifying apparatus from the top of the dehydrating column 55. Then, it is adapted such that the decomposition reaction product which is dehydrated as well as removed with CO2 by distillation in the depressurized distillation column 57 is separated into aimed decomposed and recovered product (raw material for the isocyanate compound or derivatives thereof) and undecomposed products (undecomposed compounds).
According to the apparatus for decomposing and recovering the isocyanate compound of the conventional example, an isocyanate compound having at least one isocyanate group or a group derived from an isocyanate group is brought into contact with water at high pressure and high temperature water to thereby decompose the same, whereby the raw material for the isocyanate compound or the derivative thereof can be recovered. Further, since the decomposition reaction can be conducted effectively by setting the weight of the water at high pressure and high temperature to 0.5 to 5.0 times the weight of the isocyanate compound in the reactor 51 and supplying the isocyanate compound in a molten state at 120 to 180° C. or in a solution state in which the isocyanate compound is dissolved in the solvent, the recovery rate of the raw material for the isocyanate compound or the derivative thereof is improved (for example, refer to Patent Documents 9 or 10).    Patent Document 1: JP-T-3-500264    Patent Document 2: JP-A-5-031000    Patent Document 3: JP-B-3-016328    Patent Document 4: JP-A-5-271328    Patent Document 5: GB Patent No. 0,991,387    Patent Document 6: GB Patent No. 1,047,101    Patent Document 7: U.S. Pat. No. 3,225,084    Patent Document 8: U.S. Pat. No. 4,137,266    Patent Document 9: JP-A-10-279539    Patent Document 10: U.S. Pat. No. 6,630,517